Johnsen, E., Pedersen, R., and Ona, E. 2009. Size-dependent frequency response of sandeel schools. - ICES Journal of Marine Science, 66: 1100-1105.Annual Norwegian sandeel surveys have been conducted in the North Sea since 2005 to measure the stock of lesser sandeel (Ammodytes marinus). Target identification is often a major challenge in acoustic surveys, and discriminant analyses have been used to separate echoes accurately from schools of herring, mackerel, and sandeel based on their acoustic-frequency responses measured at 18, 38, 120, and 200 kHz. At two fishing grounds during the 2008 survey, 332 schools were identified as sandeel, based on the characteristics of the acoustic signal, and validated by trawl samples. The schools consisted almost entirely of 1-year-old sandeels on one of the grounds, and 2-year-olds on the other. In this study, the potential of acoustic-frequency responses is advanced to classify the sizes of fish in them. A discriminant analysis using frequency responses as independent variables was able to differentiate between sandeel schools comprising 1- and 2-year-old fish (p < 0.001). Approximately 83% of the 2-year-old fish and 77% of the 1-year-old fish were classified correctly. The frequency responses at 18 and 38 kHz were the most important independent variables. Sandeel landings in the North Sea are normally dominated by 1- and 2-year-old sandeels in the first half of the year. This study revealed that these two age classes could be identified acoustically; a finding that may be important for acoustic surveys of sandeels and for management of the commercial sandeel fishery.

A guide to using S environments to perform statistical analyses providing both an introduction to the use of S and a course in modern statistical methods. The emphasis is on presenting practical problems and full analyses of real data sets.

Abstract Spring distribution and abundance of lesser sandeels during the day were linked to zooplankton densities, seabed substrate and various hydrographic factors using small scale empirical data collected in two areas on the Dogger Bank in 2004, 2005 and 2006. The results of a two-step generalized additive model (GAM) suggested that suitable seabed substrate and temperature best explain sandeel distribution (presence/absence) and that sandeel abundance (given presence) was best described by a...more

Acoustic surveys are used to estimate the abundance and distribution of many fish species, and have been based traditionally on data collected at a single acoustic frequency. Although it has been known for some time that the use of additional frequencies can provide information on the nature of the acoustic target, the knowledge and technology required to combine the so-called "multifrequency data" in an appropriate manner has been limited. The use of several transducers of different frequencies...more

Abstract Acoustics biomass measurements and a sandeel dredge tows were used to sample sandeels ( Ammodytes marinus ), during six surveys conducted on the Dogger Bank, North Sea between 2000 and 2002. Presented here, is an approach that uses the acoustically determined biomass to estimate the apparent efficiency of the dredge. Simulations were used to quantify the main sources of error in the estimate of acoustic biomass. The estimated dredge efficiencies varied between surveys but were comparabl...more

Tracking the movements of sandeels in relation to their habitat will provide an improved understanding of sandeel ecology, including their interrelationships with predators and fishing activity. We examined spatial and temporal changes in the behaviour and distribution of the lesser sandeel Ammodytes marinus by integrating the acoustic ground discrimination system QTC VIEW with a Simrad EK500 fisheries scientific echosounder. QTC VIEW was used to identify acoustic changes in the properties of th...more

The goal of discriminant analysis is to obtain rules that describe the separation between groups of observations. Moreover it allows to classify new observations into one of the known groups. In the classical approach discriminant rules are often based on the empirical mean and covariance matrix of the data, or of parts of the data. But because these estimates are highly influenced by outlying observations, they become inappropriate at contaminated data sets. Robust discriminant rules are obtain...more

Sandeels are not only targets of a significant industrial fishery, but are also considered to be a vital trophic component of the North Sea ecosystem. At present there is no satisfactory survey method to sample sandeels which produces a global absolute abundance estimate. Acoustic surveys have been carried out, but suffer from an inability to consistently identify sandeel echo traces in an objective manner. As sandeels lack a swimbladder, their acoustic properties are very different to other fis...more

A field experiment on the effect of seismic shooting on the lesser sandeel (Ammodytes marinus) was conducted on the fishing grounds in the southeastern part of the North Sea in May 2002. Sandeel buried in the sand at night were trapped in steel frame cages (2.0 × 1.8 × 2.0 m) deployed in positions with high abundance of fish located by a van Veen grab. In an experimental area, three sandeel cages were exposed to full-scale seismic shooting for about 2.5 d. Cameras in the cage and on a remotely o...more

The zooplanktivorous schooling fish sandeels A. marinus comprise a quarter of North Sea fish biomass and are essential food for a number of marine mammals and birds. However, in recent decades there has been a decline in the abundance of older sandeels, with increased overwinter mortality proposed as a contributor. To survive winter animals require a minimum energy reserve which is determined by temperature, prey abundance, and possibly prey size. Here a new dynamic energy budget model is create...more

Abstract Sonars and echosounders are widely used for remote sensing of life in the marine environment. There is an ongoing need to make the acoustic identification of marine species more correct and objective and thereby reduce the uncertainty of acoustic abundance estimates. In our work, data from multi-frequency echosounders working simultaneously with nearly identical and overlapping acoustic beams are processed stepwise in a modular sequence to improve data, detect schools and categorize aco...more

There is an increased need to detect, identify, and monitor natural and manmade seabed gas leaks. Fisheries echosounders are well suited to monitor large volumes of water and acoustic frequency response [normalized acoustic backscatter, when a measure at one selected frequency is used as a denominator, r(f)] is commonly used to identify echoes from fish and zooplankton species. Information on gas plume r(f) would be valuable for automatic detection of subsea leaks and for separating bubble plume...more

Fisheries independent monitoring of widely distributed pelagic fish species which conduct large seasonal migrations is logistically complex and expensive.One of the commercially most important examples of such a species in the Northeast Atlantic Ocean is mackerel for which up to recently only an international triennial egg survey contributed to the stock assessment. In this study, we explore whether fisheries acoustic data, recorded opportunistically during the English component of the North Sea...more